(a) Field of the Invention
The invention relates to a source driver of a thin film transistor liquid crystal display (TFT-LCD) and, more particularly, to a TFT-LCD source driver having a built-in test circuit.
(b) Description of the Related Art
Nowadays, the mass production test for an LCD source driver is performed by a tester. FIG. 1 shows a schematic diagram illustrating a conventional source driver 10 consisting of N driving units 11. Each driving unit 11 includes a level shifter 111, a digital to analog converter (DAC) 112, and a unity gain buffer 113. Digital data received by each driving unit 11 is modified by the level shifter 111 and then transmitted to the DAC 112. The DAC 112 outputs analog output signals via the unity gain buffer 113; hence, a typical source driver may generate output signals S(1)–S(N), as shown in FIG. 1. FIG.2 shows a schematic diagram illustrating a conventional tester 20 for testing the source driver 10. A typical tester 20 includes P test units 21, and each unit consists of a multiplexer 211 and an analog to digital converter (ADC) 212. Each multiplexer 211 receives M analog output signals S(1)–S(M). Note that the numbers of the P test units 21, M analog output signals received by the multiplexer 211, and N driving units 11 must satisfy the condition P×M≧N.
In this embodiment, the tester 20 receives N analog output signals S(1)–S(N) output from the source driver 10, and each multiplexer 211 in the P test units 21 receives M analog output signals S(1)–S(M). The multiplexer 211 selects one of the analog output signals S(1)–S(M) as a test signal through the control of a select signal, and then the test signal is transmitted to the ADC 212 to be transformed into digital data. Finally, the tester 20 may judge whether the output voltage of the source driver 10 conforms to a specification according to all digital data transformed from the ADC 212 to completely examine the characteristic of the source driver 10.
However, the number of output pins in a typical source driver often ranges from 300 to 500; in other words, the number N of the drive units 11 equals approximately 300–500. To satisfy the condition P×M≧N for the tester design, the number M of input pins of one test unit 21 (equal to the number M of the analog output signals received by one multiplexer 211) and the number P of the test units 21 must be increased as the number N of the drive units 11 is increased. Under the circumstance, the increase in layout areas for the total input pins of the tester 20 and the number of the test units may result in a considerable occupied space of the tester. Additionally, in that case, the ADC 212 is required to have a high resolution to meet the measure requirement of a high accuracy, so that the tester 20 incorporating the ADC 212 is expensive. For these reasons, the cost of testing an LCD source driver is high.
Hence, a solution to reduce the occupied space of a tester and the testing cost of an LCD source driver and to provide a highly accuracy measurement is urgently needed.